The present invention relates generally to the field of electronic data storage and retrieve systems. In particular, the present invention relates to a magnetoresistive read sensor having improved thermal and magnetic stability.
In an electronic data storage and retrieval system, a transducing head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically-encoded information stored on a magnetic disc. MR sensors may be anisotropic magnetoresistive (AMR) sensors or giant magnetorisistive (GMR) sensors. AMR sensors generally have a single MR layer formed of a ferromagnetic material. GMR sensors generally have multiple layers or a “stack” of ferromagnetic material.
A GMR stack generally consists of a free layer and a pinned layer, which are comprised of magnetic material. The pinned layer has a magnetization vector that is pinned and cannot rotate. The free layer on the other hand, has a magnetization vector that is allowed to rotate relative to the pinned layer. When a GMR sensor is placed in close proximity to a rotating magnetized storage disc, the free layer is exposed to magnetic bit fields previously written on the magnetic surface. Exposing the free layer to the magnetic bit fields in this way, affects the magnetization vector of the free layer, which then rotates relative to the magnetization vector of the pinned layer. When a current is passed through the GMR stack, changes in resistance are detected as voltage changes. The change in resistance is due to the changing magnetization vector of the free layer relative to the pinned layer. External circuitry then converts the voltage information into an appropriate format and manipulates that information into a series of binary ones and zeros that represent the recorded bits on the storage disc.
Information that is read by the GMR sensor is initially stored on the magnetic discs in tracks. The magnetic recording industry has been continually doubling the amount of information stored in a unit area of the magnetic disc for a prolonged period of time. A measure of the amount of information stored in a unit area is often referred to as the areal density. The areal density is a reflection of the number of bits written in a particular unit area. In order to accommodate ever increasing areal density, improvements have been made in the magnetic media, and transducing heads have been decreased in size.
Even though transducing head dimensions have decreased by orders of magnitude, the output signals have been required to remain consistent. In order to obtain consistent read-back amplitudes with much smaller transducers, GMR sensors with much higher GMR ratios have been required. Also, thickness of the reader gaps in the GMR sensors has been decreased. Similarly, the free layer thickness and corresponding permanent magnet biasing element thickness has been decreasing to provide more free layer magnetization rotation and larger efficiency.
Even with these significant design and process improvements, improved designs are needed to provide consistent output signals, increase stability of MR sensors, and decrease the overall head size.